1 Technical Field
The invention relates generally to a spark plug for an internal combustion engine used in, for example, an automotive vehicle.
2 Background Art
Some of spark plugs used as an ignition device for spark-ignition engines such as internal combustion engines mounted in automotive vehicles are equipped with a center electrode and a ground electrode which face each other in an axial direction of the park plug to define a discharge gap (also called a spark gap). This type of spark plugs work to create a sequence of sparks in the spark gap to ignite an air-fuel mixture in a combustion chamber of the engine.
Typically, flows of the mixture, such as swirls or tumbles, are created within the combustion chamber. Such mixture flows also pass through the spark gap to ensure ignition of the mixture in the combustion chamber. A portion of the ground electrode joined to a head of a housing (also called a metal shell) of the spark plug may, however, be located on an upstream side of the spark gap in the mixture flow depending upon an orientation of the spark plug mounted in the internal combustion engine (which will also be referred to below as mounting orientation). The portion of the ground electrode, thus, obstructs the mixture flow in the combustion chamber, which may lead to a risk that the mixture flow is stagnated around the spark gap, which results in a deterioration of ignition ability of the spark plug. In other words, the spark plugs have a drawback that the ignition ability varies depending upon the mounting orientation thereof. In recent years, lots of lean-burn internal combustion engines become frequently used. Such types of engines have a risk that the mounting orientation of the spark plug results in instability of combustion of the mixture.
Usually, it is difficult for internal combustion engines to control the mounting orientation of the spark plug, that is, the location of the ground electrode in a circumferential direction of the spark plug. This is because the mounting orientation of the spark plug usually depends upon orientation of an external thread formed on the housing of the spark plug or the degree to which the spark plug is fastened into the internal combustion engine in a spark plug mounting operation.
Japanese Patent First Publication No. 9-148045 teaches a spark plug designed to have a ground electrode in which a hole is formed or which is welded to the housing through a plurality of thin plates in order to eliminate the problem that the ground electrode obstructs the mixture flow.
The hole formed in the ground electrode of the spark plug may, however, result in a decrease in mechanical strength of the ground electrode. Thickening the ground electrode to ensure the mechanical strength thereof will facilitate interruption of the mixture flow.
Further, the use of the thin plates through which the ground electrode is welded to the housing results in a complicated configuration of the ground electrode, which will lead to an increased number of production processes or increased production cost of the spark plug.